This invention relates to an improved process for reducing sulfur content of solid carbonaceous fuels, such as coal and lignite, to provide gas, liquid, and solid fuel products which can be burned without further treatment or use of special devices to produce stack gases which meet pollution regulations regarding sulfur emissions. More particularly, it relates to the removal of pyritic and organic sulfur from coal without added hydrogen.
Coal constitutes the largest single fossil fuel source of the United States. Nevertheless, its use has been restricted because of environmental regulations and the availability of petroleum and natural gas as alternative sources of energy. However, to achieve the national goal of energy self-sufficiency it is now apparent that coal must play a major role in meeting our expanding energy requirements. A principal drawback to the use of United States coal is the sulfur content, which can range up to 5 percent or more; large quantities of sulfur compounds, which have been found to be environmentally hazardous, are discharged into the atmosphere when coal is burned to produce energy.
A variety of methods have been suggested to reduce the discharge of such sulfur compounds into the atmosphere when sulfur-containing fuels such as coal are burned. Two general methods have been tried. One method involves removing sulfur from stack gases after the sulfur-containing fuel is burned, whereas the other method removes sulfur from the fuel before it is burned. While numerous methods have been tried for stack gas cleaning, none appear to be simple or low cost. The inherent difficulties of such an approach are the enormous volumes of stack gas that must be processed and the low concentration of sulfur in these gases.
It is desirable to reduce the sulfur content of the coal initially. If this is successful, then the fuel can be burned as it has been in the past -- i.e. without material change in the operation of furnaces, boilers, and utility plants. Moreover, sulfur removal may be accomplished at one location without the need to provide extensive sulfur removal facilities at each location. Accordingly, it is very desirable to be able to substantially reduce the sulfur content of a coal before it is burned as a fuel or otherwise gasified and/or liquified for further processing into specific fuels.
The solvent-refined coal or coal extraction process reduces the sulfur content of coal by first dissolving the coal in a suitable solvent to produce a mixture of liquid and undissolved solids from which the solid may be removed by filtration or other conventional solids -- liquid separation processes. The dissolution step is often carried out under hydrogen pressure. Solvent is recovered from the filtrate by means such as vacuum distillation. The distillation residue can be handled in either solid or liquid form and is a low ash, low sulfur material known as solvent refined coal or coal extract. However, such a process has relatively expensive and complex equipment requirements. Furthermore, considerable difficulty has been encountered in the solvent-extract/undissolved residue separation step. Finally, although such a process converts a majority of the coal feed to a low-sulfur fuel product, the process is inefficient in terms of the quantity of incoming hydrocarbonaceous material converted. For example, extract yields normally approach 80 percent of the moisture and ash-free coal feed.
A variation of the solvent refined coal process is the coal liquefaction process which is characterized by the attempt to completely hydrogenate or liquify coal to produce an oil-like product very low in sulfur. The process is subject to the same disadvantages as the solvent refined coal process, referred to above; and, in addition, the operating costs of the process are exceptionally high because of the large hydrogen requirements.
The use of fluidized systems wherein a fluidized stream of finely divided coal particles and/or heated char particles is formed in a carrier stream to pyrolyze the coal particles, extracting the volatiles therefrom, is well known in the art. The heated char particles and/or the carrier gas stream are utilized to provide the requisite heat of pyrolysis to the coal particles. A supply of heated char is continuously produced upon pyrolysis of the coal in the system. Sulfur contaminants may be removed by the addition of sulfur acceptors such as iron oxides or lime to the particulate coal prior to processing or by heating the products to high temperatures in the presence of hydrogen upon removal of the products from the pyrolysis zone. Alternatively, desulfurization may be achieved during pyrolysis by enriching the carrier gas stream with hydrogen, which may be generated within the process by known gasification methods. Exemplary of such systems are: U.S. Pat. Nos. 3,007,849; 3,702,516; 3,736,233. Additional references relating to the pyrolysis method which are considered of some pertinency are found in Coal Processing Technology, Vol. 2, American Institute of Chemical Engineers, New York, N.Y. (1975), pp. 83-93, 119-120.
Another method employed to reduce the sulfur content of high-sulfur coal is the gasification of coal with steam and air or oxygen to produce fuel gas which must then be desulfurized prior to combustion. For example, U.S. Pat. No. 2,634,286, teaches hydrogenation of coal in the dry state by passing a stream of heated, hydrogen-containing gas upwardly through a reaction zone containing a mass of the substantially dry coal particles at a velocity sufficient to fluidize the mass. The reaction zone is maintained under an elevated temperature (450.degree. to 650.degree. C.) and pressure (250 to 1500 psi). The hydrogen and the coal react to produce a major proportion of liquid hydrocarbons and a minor proportion of gaseous hydrocarbons together with a finely-divided, solid, low-sulfur content char. Processes similar to U.S. Pat. No. 2,634,286 are also discussed in Coal Processing Technology, Vol. 2, American Institute of Chemical Engineers, New York, N.Y. (1975), pp. 88-93 and 119-120.
The present invention differs from the gasification process referred to above in that the production of gaseous fuels is not maximized. Rather, only a minimum amount of coal is gasified and converted to liquid and gas products.
U.S. Pat. No. 3,909,212 issued to Schroeder discloses the concept of producing a low-sulfur solid fuel from coal with minimum chemical change in the coal without added hydrogen. The primary fuel product is not a coke or char; the feed coal is carbonized only to the extent necessary to remove a substantial amount of the sulfur from the coal. The desulfurized coal still contains sufficient volatile matter to be a satisfactory fuel for combustion purposes. The process comprises reacting particulate coal under a pressure of at least two atmospheres with an oxygen-containing gas and steam in a reaction zone. The amount of oxygen-containing gas is just sufficient to burn that portion of the particulate coal which will raise the temperature of the coal in the reaction zone to about 1100.degree. to 1500.degree. F. The amount of steam is sufficient to react with the fuel particles to generate nascent hydrogen which reacts with the sulfur in the fuel particles to form hydrogen sulfide. In a preferred embodiment of the invention, the amount of hydrogen, including that generated by the steam reaction and that present in the coal, is about 4 to 5 times that theoretically required for reaction with all of the sulfur in the coal to produce H.sub.2 S.